Cardiomyocyte T-tubules are important for regulating ionic flux. Bridging Integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is down-regulated in failing hearts. Here we find that cardiac T-tubules normally contain dense protective inner membrane folds that are formed by a cardiac spliced isoform of BIN1. In mice with cardiac Bin1 deletion, T-tubule folding is decreased which does not change overall cardiomyocyte morphology, but frees diffusion of local extracellular calcium and potassium ions, prolonging action potential duration, and increasing susceptibility to ventricular arrhythmias. We also find that T-tubule inner folds are rescued only by the BIN1 isoform BIN1+13+17, which promotes N-WASP dependent actin polymerization to stabilize T-tubule membrane at cardiac Z-discs. In conclusion, BIN1+13+17 recruits actin to fold T-tubule membrane, creating a fuzzy space that protectively restricts ionic flux. When BIN1+13+17 is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered and arrhythmias can result.

Altered phosphorylation and trafficking of connexin 43 (Cx43) during acute ischemia contributes to arrhythmogenic gap junction remodeling, yet the critical sequence and accessory proteins necessary for Cx43 internalization remain unresolved. 14-3-3 proteins can regulate protein trafficking, and a 14-3-3 mode-1 binding motif is activated upon phosphorylation of Ser373 of the Cx43 C-terminus. We hypothesized that Cx43Ser373 phosphorylation is important to pathologic gap junction remodeling. Immunofluorescence in human heart reveals enrichment of 14-3-3 proteins at intercalated discs, suggesting interaction with gap junctions. Knockdown of 14-3-3τ in cell lines increases gap junction plaque size at cell-cell borders. Cx43S373A mutation prevents Cx43/14-3-3 complexing and stabilizes Cx43 at the cell surface, indicating avoidance of degradation. Using Langendorff-perfused mouse hearts we detect phosphorylation of newly internalized Cx43 at Ser373 and Ser368 within 30 minutes of no-flow ischemia. Phosphorylation of Cx43 at Ser368 by PKC and Ser255 by MAPK has previously been implicated in Cx43 internalization. The Cx43S373A mutant is resistant to phosphorylation at both these residues and does not undergo ubiquitination, revealing Ser373 phosphorylation as an upstream gate-keeper of a post-translational modification cascade necessary for Cx43 internalization. Cx43Ser373 phosphorylation is a potent target for therapeutic interventions to preserve gap junction coupling in the stressed myocardium.